The known devices of this type generally use a mask having a centric Fresnel configuration. Although perfectly suitable for the case of punctiform objects such as stars, these known devices give rise to numerous difficulties in the case of continuous objects such as human organs which have been marked with radioactive tracers. In particular, the centric Fresnel region leads to the superimposition of the various diffracted orders. In order to remedy these difficulties, it has already been proposed to use, as a coding screen or mask, an eccentric portion of a Fresnel configuration. However, such an incoherent holographic device behaves, from the point of view of imaging, as a pass-band filter cutting off the elements of low spatial frequency. It then becomes necessary to arrange, either on or in the vicinity of the object whose image is desired, a second mask which modulates the object to the mean frequency transmitted by the Fresnel region.
The invention eliminates these disadvantages and, in particular, it has as its object the provision of a device in which a single mask is used.